Regenerative amplifiers (RA) are used to amplify optical signals in order to increase pulse energy. Optical signals to be amplified are transmitted and locked inside regenerative amplifier by means of switching electro-optical elements, such as Pockels cells, driven by high voltage, typically in the kilovolt range. Said input optical signals are referred to as seed pulses.
The gain media inside RA is pumped by a light source, such as laser diodes or flash lamps. The pump radiation excites atoms inside the gain medium and passing seed pulses induce stimulated emission of radiation and thus are amplified.
In conventional RA designs, polarizing electro-optical elements, such as Pockels cells are arranged inside a resonator in order to act as polarization rotators or polarization state converters, whenever a seed pulse is to be locked inside RA cavity for amplification. Electro-optical elements are controlled by means of rapid voltage change. Absence of voltage means that the electro-optical element is not working, while application of voltage results in rotation or change of polarization state of a seed wave. Reverse operation is also possible, when inactive Pockels cell rotates the polarization and whenever it is activated by application of voltage, the resulting polarization of the optical field remains unchanged as compared to the polarization of the incident beam.
Another electro-optical or acousto-optical device is usually arranged on the output of the RA, in order to act as a pulse picker. It works in combination with a polarizer mirror and based on the resulting polarization of the optical field, amplified pulses are directed towards a laser output or towards a beam dump.
A US patent No. US2004075892, published on Apr. 22, 2004, describes a regenerative optical amplifier enabling voltage to be easily applied to polarizing elements such as Pockels cells, without the need for complicated drive circuitry. An input beam of S-polarized light is reflected by a polarizer and advances to a Pockels cell. During the time, which it takes for the input pulse, having once passed through the Pockels cell, to be reflected by a reflective mirror and return to the Pockels cell, a voltage causing a 90-degree rotation in the polarization of transmitted light is applied to the Pockels cell, and this applied voltage is maintained. The input beam is converted by the Pockels cell into a P-polarized light pulse which is transmitted by the polarizer. Subsequently, the light pulse is converted from P-polarized light to S-polarized light and back to P-polarized light with each round-trip of the Pockels cell, while passing each time between the reflective mirror, laser crystal and other reflective mirror, so as to be amplified in the resonator formed thereby. The amplified light pulse is extracted by applying a voltage VP2 causing a 90-degree rotation of the polarization of the transmitted light to the Pockels cell to convert the light pulse to S-polarized light which is then reflected out of the resonator by the polarizer.
A PCT application No. W02005069963, published on Aug. 4, 2005 describes a regenerative amplifier system that is optimized for low-gain gain media is provided. The system is configured to include a minimum number of intra-cavity elements while still eliminating the leakage of the seed pulses from the output beam. In addition, the contrast ratio of the amplified pulses is increased even considering the long build-up time that is required in low-gain regenerative amplifiers. This is accomplished using a single Pockels cell between the oscillator and amplifier to select a single seed pulse for the cavity, instead of using a Faraday isolator. This directs the unwanted seed pulses in a separate direction from the output pulse. When the amplified pulse exits the cavity, it is directed in a direction away from the oscillator by the same Pockets cell. Only one additional Pockels cell and one polarizer are required inside the regenerative amplifier cavity.
An European patent, No. EP1801635, published on Jun. 27, 2007 describes a controllable Pockels cell system, which has a switching unit that can apply voltage to the Pockels cell. The Pockels cell system also features a delay unit that enables setting of a precise time when voltage is applied or removed from the Pockels cell. This allows displacing in time the voltage pulse applied to the Pockels cell, in this manner also displacing in time the transmission pulse of the Pockels cell with an analyzer located behind the Pockels cell. Thus it is possible to individually control the amplitude of selected laser pulses. The switching unit can either be a simple push-pull circuit or a bridge circuit made from two push-pull circuits.
Using several Pockels cells significantly increases the prime cost of the regenerative amplifier. Synchronization, simultaneous control of several electro-optical devices and monitoring makes the control electronics more complex. Extra heat is generated by the driving circuitry of the additional Pockels cell running at high repetition rates, which reduces the efficiency of the laser system.
Majority of prior art inventions and technical solutions use single optical path for input of the seed beam and output of the amplified beam. At some point in the optical pathway, outside the RA resonator, seed and amplified pulse beams are separated by means of a Pockels cell or a Faraday isolator. A drawback of such optical design is that some losses of the pulse energy occur, when the amplified pulse passes the Pockels cell on the output of the RA, as well as excess heat is generated in the crystal of the Pockels cell. Such layout is rather inefficient and might cause additional distortions of the beam or pulse shape.